Getting Closer to Solving Combustion Mystery

When most people look at a flame, they likely do not think about the many chemical processes involved. For those that do consider the mechanics of combustion though, there has been a mystery for some time about how soot and compounds are formed. Researchers at Berkeley Lab and the University of Hawaii though have finally found the first step in the process, which could lead to many interesting and useful discoveries.

Under ideal conditions, combustion should just create carbon dioxide and water vapor. Reality is hardly ideal though, and so we see fumes and particulates, such as soot, created as well. The mechanisms that convert gas-phase molecules into these solid particles have been theorized for some time, but now the researchers have experimental evidence for what happens. One family of theories called HACA (hydrogen abstraction-acetylene addition) involves benzene molecules, which are a ring of six carbon atoms with six hydrogen atoms connected, that lose a hydrogen atom and have a tail of acetylene take its spot. Another acetylene molecule eventually attaches to the first, making the tail long enough to curl around and attach to the original benzene ring. This double-ring molecule, called naphthalene, is what eventually develops into soot and other macro molecules.

To identify the mechanism, the researchers created a combustion environment and let the process start before sending the molecules into a mass spectrometer that identified the naphthalene and benzene with acetylene tail. This supports the version of HACA that has the benzene forming a single tail, instead of multiple.

How this matters is that by better understanding the real-world mechanisms of combustion, it may be possible to develop fuels that burn more closely to ideal, making them more efficient and cleaner. Also these mechanisms have implications for how gases emitted by stars can become the carbon-based matter we find in space.